FIG. 4 is a section of a conventional speaker. Voice coil 22 is disposed in the gap of magnetic circuit 21. The inner circumferential edge of diaphragm 23 is connected to voice coil 22. The outer circumferential edge of diaphragm 23 is coupled to frame 25 via edge 24. The backside of diaphragm 23 is coupled to frame 25 via suspension holder 26 and edge 27. The symmetrical structure of edges 24 and 27 allows diaphragm 23 to vibrate as symmetrically as possible in the vertical direction, thus reducing distortion of the speaker. This conventional art is disclosed in Japanese Patent Unexamined Publication No. 2004-7332, for example.
However, in the conventional speaker, ring-shaped edge 27 has diameter a smaller than diameter b of ring-shaped edge 24; thus, the speaker cannot eliminate harmonic distortion sufficiently.